Update the JOSS plotting scripts and design plots

[NicolaCourtier]

Description

Update the JOSS plotting scripts to work with the current version of PyBOP and re-create the design optimisation plots in light of a bug fix applied in PR 857 (January 2026) to the pybop.pybamm.Simulator class.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.95%. Comparing base (3171a07) to head (62fd833).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #966      +/-   ##
===========================================
+ Coverage    92.13%   92.95%   +0.81%     
===========================================
  Files           74       80       +6     
  Lines         5799     6614     +815     
===========================================
+ Hits          5343     6148     +805     
- Misses         456      466      +10     

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